Blow molding of plastic parts is a mature techology. The basic process involves extrusion of a tubular molten mass of plastic called a parison, enclosing the extruded tubular parison while still in a semi-fluid state in a mold, and expanding the parison by creating a pressure differential between the parison inside and the parison outside into contact with the mold. The solidified expanded parison, now a blow molded article, is removed from the mold and may be further processed.
Uneven wall thickness distribution of blow molded articles has been a problem in the blow molding technology. One solution to solve this problem has been die shaping. Die shaping involves cutting grooves or indentations (called shapes) on the land area (die area located upstream of the die opening) of the bushing and/or the mandrel at one or more locations such as to produce a parison having uneven wall thickness distribution, the thicker wall material being located at an area of the parison which corresponds to the area of the molded article wherein undesirably thin wall section distributions have been encountered prior to the shaping. The more complex a mold is, that is the more a mold deviates in cross section from a circular cross section, the more significant the uneveness of the wall thickness of the blow molded articles becomes.
The known shaping procedures are basically empirical. Trial and error techniques such as crayon marking on the molten parison while hanging between operating mold halves in order to determine the precise location of the required shaped areas of a molded article relative to the parison have been used. The degree of shaping, and particularly the depth, the width and the length of the shaping are also based on trial and error. For every new mold and for every new die these values had again to be determined in a time consuming, frustrating and expensive procedure.